Single crystal, dual wafer, tunneling sensor and a method of making same

ABSTRACT

A method of making a micro electromechanical switch or tunneling sensor. A cantilevered beam structure and a mating structure are defined on a first substrate or wafer; and at least one contact structure and a mating structure are defined on a second substrate or wafer, the mating structure on the second substrate or wafer being of a complementary shape to the mating structure on the first substrate or wafer. A bonding layer, preferably a eutectic bonding layer, is provided on at least one of the mating structures. The mating structure of the first substrate is moved into a confronting relationship with the mating structure of the second substrate or wafer. Pressure is applied between the two substrates so as to cause a bond to occur between the two mating structures at the bonding or eutectic layer. Then the first substrate or wafer is removed to free the cantilevered beam structure for movement relative to the second substrate or wafer.

TECHNICAL FIELD

[0001] The present invention relates to micro electro-mechanical (MEM)tunneling sensors using dual wafers which are bonded together preferablyeutectically.

RELATED APPLICATIONS

[0002] This invention is related to other inventions which the subjectof separate patent applications filed thereon. See: U.S. patentapplication Ser. No. ______ entitled “A Single Crystal, Dual Wafer,Tunneling Sensor or Switch with Silicon on Insulator Substrate and aMethod of Making Same” (attorney docket 617965-3) and U.S. patentapplication Ser. No. ______ entitled “A Single Crystal, Dual Wafer,Tunneling Sensor or Switch with Substrate Protrusion and a Method ofMaking Same” (attorney docket 617337-2), both of which applications havethe same filing date as this application. The inventors named in thisapplication are the inventors of the first embodiment disclosed hereinwith reference to FIGS. 1A-12B. However, subsequent improvements havebeen made which reflect what is believed to be the best modes forpracticing the invention. As such, this application includes adisclosure of those improvements in the embodiments after FIG. 12B toensure that the best mode requirements of 35 USC 111 first paragraphhave been satisfied.

BACKGROUND OF THE INVENTION

[0003] The present invention provides a new process of fabricating asingle crystal silicon MEM tunneling devices using low-cost bulkmicromachining techniques while providing the advantages of surfacemicromachining. The prior art, in terms of surface micromachining, usese-beam evaporated metal that is patterned on a silicon dioxide (SiO₂)layer to form the control, self-test, and tip electrodes of a tunnelingMEM sensor. A cantilevered beam is then formed over the electrodes usinga sacrificial resist layer, a plating seed layer, a resist mold, andmetal electroplating. Finally, the sacrificial layer is removed using aseries of chemical etchants. The prior art for bulk micromachining hasutilized either mechanical pins and/or epoxy for the assembly ofmulti-Si wafer stacks, a multi-Si wafer stack using metal-to-metalbonding and an active sandwiched membrane of silicon nitride and metal,or a dissolved wafer process on quartz substrates (Si-on-quartz) usinganodic bonding. None of these bulk micromachining processes allow one tofabricate a single crystal Si cantilever (with no deposited layers overbroad areas on the beam which can produce thermally mismatched expansioncoefficients) above a set of tunneling electrodes on a Si substrate andalso electrically connect the cantilever to pads located on thesubstrate. The fabrication techniques described herein provide thesecapabilities in addition to providing a low temperature process so thatCMOS circuitry can be fabricated in the Si substrate before the MEMSsensors are added. Finally, the use of single crystal Si for thecantilever provides for improved process reproductibility forcontrolling the stress and device geometry.

[0004] Tunneling sensors may be used in various military, navigation,automotive, and space applications. Space applications include satellitestabilization in which MEM sensor technology can significantly reducethe cost, power, and weight of the presently used gyro systems.Automotive air bag deployment, ride control, and anti-lock brake systemsprovide other applications for MEM sensors. Military applicationsinclude high dynamic range accelerometers and low drift gyros.

BRIEF DESCRIPTION OF THE INVENTION

[0005] Generally speaking, the present invention provides a method ofmaking a micro electromechanical sensor wherein a cantilevered beamstructure and a mating structure are defined on a first substrate orwafer and at least one contact structure and a mating structure aredefined on a second substrate or wafer. The mating structure on thesecond substrate or wafer is of a complementary shape to the matingstructures on the first substrate or wafer. A bonding or eutectic layeris provided on at least one of the mating structures and the matingstructure are moved into a confronting relationship with each other.Pressure is then applied between the two substrates and heat may also beapplied so as to cause a bond to occur between the two mating structuresat the bonding or eutectic layer. Then the first substrate or wafer isremoved to free the cantilevered beam structure for movement relative tothe second substrate or wafer. The bonding or eutectic layer alsoprovides a convenient electrical path to the cantilevered beam formaking a circuit with the contact formed on the cantilevered beam.

[0006] In another aspect, the present invention provides an assembly orassemblies for making a single crystal silicon MEM sensor therefrom. Afirst substrate or wafer is provided upon which is defined a beamstructure and a mating structure. A second substrate or wafer isprovided upon which is defined at least one contact structure and amating structure, the mating structure on the second substrate or waferbeing of a complementary shape to the mating structure on the firstsubstrate or wafer. A pressure and heat sensitive bonding layer isdisposed on at least one of the mating structures for bonding the matingstructure defined on the first substrate or wafer with the matingstructure on the second substrate in response to the application ofpressure and heat therebetween.

BRIEF DESCRIPTION OF THE FIGURES

[0007]FIGS. 1A through 6A depict the fabrication of a first embodimentof the cantilever portion of a MEM sensor.

[0008]FIGS. 1B through 6B correspond to FIGS. 1A-6A, but show thecantilever portion, during its various stages of fabrication, in planview:

[0009]FIGS. 7A through 9A show, in cross section view, the fabricationof the base portion of the first embodiment tunneling sensor;

[0010]FIGS. 7B through 9B correspond to FIGS. 7A-9A but show thefabrication process for the base portion in plan view;

[0011]FIGS. 10 and 11 show the cantilever portion and the base portionbeing aligned with each other and being bonded together preferably byeutectic bonding;

[0012]FIGS. 12A and 12B show in a cross sectional view and in a planview the completed tunneling sensor according to the first embodiment ofthe invention:

[0013]FIGS. 13A and 14A depict steps used in fabricating a secondembodiment of a the cantilever portion of a MEM sensor;

[0014]FIGS. 13B and 14B correspond to FIGS. 13A and 14A, but show thecantilever portion, in plan view;

[0015] FIGS. 15A-19A depict, in cross section view, the fabrication ofthe base portion of the second embodiment of the tunneling sensor;

[0016] FIGS. 15B-19B correspond to FIGS. 15A-19A, but show thefabrication process for the second embodiment of the wafer in plan view;

[0017]FIGS. 20 and 21 show the cantilever and base portion embodimentbeing aligned with each other and bonded together preferably by eutecticbonding;

[0018]FIGS. 22A and 23 show the completed MEM sensor according to thesecond embodiment in cross sectional view, while FIG. 22B shows thecompleted MEM sensor according to the second embodiment in plansectional view;

[0019]FIGS. 24A through 29A depict, in cross sectional view, amodification applicable to both the first and second embodiments of thecantilever portion of the MEM sensor;

[0020]FIGS. 24B through 29B correspond to FIGS. 24A-29A, but show thefabrication process for the modification in plan view;

[0021]FIG. 30 depicts a side elevational section view of anotherembodiment of a MEM sensor, this embodiment having a preferably eutecticbond in a central region of its columnar support;

[0022]FIG. 31 depicts a side elevational section view of yet anotherembodiment of a MEM sensor, this embodiment having a preferably eutecticbond adjacent the cantilevered beam 12;

[0023]FIG. 32 depicts a side elevational section view of still anotherembodiment of a MEM sensor, this embodiment having a preferably eutecticbond in a central region of its columnar support as in the embodiment ofFIG. 30, but also having a ribbon conductor on the cantilevered beamstructure;

[0024]FIG. 33 depicts a side elevational section view of anotherembodiment of a MEM sensor, t this embodiment having a preferablyeutectic bond adjacent the cantilevered beam structure as in the case ofthe embodiment of FIG. 31, but also having a ribbon conductor on thecantilevered beam structure;

[0025]FIG. 34 depicts a side elevational section view of still anotherembodiment of a MEM sensor, this embodiment having a preferably eutecticbond adjacent the cantilevered beam, but also utilizing a base structurehaving a silicon protrusion which forms part of the columnar supportstructure;

[0026]FIG. 35 depicts a side elevational section view of yet anotherembodiment of a MEM sensor, this embodiment having a preferably eutecticbond adjacent the cantilevered beam and utilizing a base structurehaving a silicon protrusion which forms part of the columnar supportstructure as in the case of the embodiment of FIG. 34, but alsoutilizing a ribbon conductor on the cantilevered beam structure;

[0027]FIG. 36 depicts a side elevational section view of anotherembodiment of a MEM sensor, this embodiment having a preferably eutecticbond in a central region of its columnar support as in the embodiment ofFIG. 30, but also utilizing a base structure having a silicon protrusionwhich forms part of the columnar support structure;

[0028]FIG. 37 depicts a side elevational section view of anotherembodiment of a MEM sensor, this embodiment having a preferably eutecticbond in a central region of its columnar support and a base structurehaving a silicon protrusion which forms part of the columnar supportstructure as in the embodiment of FIG. 36, but also utilizing a ribbonconductor on the cantilevered beam structure;

[0029]FIG. 38 depicts a side elevational section view of an embodimentof a MEM switch, this embodiment being similar to the sensor embodimentof FIG. 32, but being equipped with an additional pad which is used toapply electrostatic forces to the beam to close the switch;

[0030]FIG. 39 depicts a side elevational section view of anotherembodiment of a MEM switch, this embodiment being similar to the switchembodiment of FIG. 38, but the preferably eutectic bond occurs adjacentthe cantilevered beam as opposed in a central region of the columnarsupport;

[0031]FIG. 40 depicts a side elevational section view of yet anotherembodiment of a MEM switch, this embodiment being similar to the switchembodiment of FIG. 39, but also utilizing a base structure having asilicon protrusion which forms part of the columnar support structurefor the cantilevered beam; and

[0032]FIG. 41 depicts a side elevational section view of yet anotherembodiment of a MEM switch, this embodiment being similar to the switchembodiment of FIG. 40, but including an SiO₂ layer between the ribbonconductor and the Si of the cantilevered beam.

DETAILED DESCRIPTION

[0033] Several embodiments of the invention will be described withrespect to the aforementioned figures. The first embodiment will bedescribed with reference to FIGS. 1A through 15. A second embodimentwill be discussed with reference to FIGS. 16 through 23. Furtheradditional embodiments and modifications are described thereafter. Sincesome of the fabrication steps are the same for many of the embodiments,reference will often be made to earlier discussed embodiments to reducerepetition. For example, the second embodiment makes reference to FIGS.1A-4B when describing the second embodiment to reduce repetition of thatmaterial.

[0034] The MEM devices shown in the accompanying figures are not drawnto scale, but rather are drawn to depict the relevant structures forthose skilled in this art. Those skilled in this art realize that thesedevices, while mechanical in nature, are very small and are typicallymanufactured using generally the same type of technology used to producesemiconductor devices. Thus a thousand or more devices might well bemanufactured at one time on a silicon wafer. To gain an appreciation ofthe small scale of these devices, the reader may wish to turn to FIG. 15which includes size information for a preferred embodiment of a MEMsensor utilizing the present invention.

[0035] Turning to FIGS. 1A and 1B, a starting wafer for the fabricationof the cantilever is depicted. The starting wafer includes a wafer ofbulk n-type silicon (Si) 10 upon which is formed a thin layer of dopedp-type silicon 12. The silicon wafer 10 is preferably of a singlecrystalline structure having a <100> crystalline orientation. The p-typesilicon layer 12 is preferably grown as an epitaxial layer on siliconwafer 10. The layer 12 preferably has a thickness of in the range of 1to 20 micrometers (μm), but can have a thickness anywhere in the rangeof 0.1 μm to 800 μm. Generally speaking, the longer the cantileveredbeam is the thicker the beam is. Since layer 12 will eventually form thecantilevered beam, the thickness of layer 12 is selected to suit thelength of the beam to be formed.

[0036] Layer 12 is doped with Boron such that its resistivity is reducedto less than 0.05 Ω-cm and is preferably doped to drop its resistivityto the range of 0.01 to 0.05 Ω-cm. The resistivity of the bulk siliconwafer or substrate 10 is preferably about 10 Ω-cm. Boron is a relativelysmall atom compared to silicon, and therefore including it as a dopantat the levels needed (10²⁰) in order to reduce the resistivity of thelayer 12 tends to induce stress which is preferably compensated for byalso doping, at a similar concentration level, a non-impurity atomhaving a larger atom size, such as germanium. Germanium is considered anon-impurity since it neither contributes nor removes any electroncarriers in the resulting material.

[0037] Layer 12 shown in FIGS. 1A and 1B is patterned using well knownphotolithographic techniques to form a mask layer, patterned as shown atnumeral 14, preferably to assume the shape of a capital letter ‘E’.While the shape of the capital letter ‘E’ is preferred, other shapes canbe used. In this embodiment, the outer peripheral portion of the E-shapewill form a mating structure which will be used to join the cantileverportion of the sensor to the base portion.

[0038] After the mask layer 14 has been patterned as shown in FIGS. 2Aand 2B, the wafer is subjected to a plasma etch in order to etch throughthe thin layer of p-type doped silicon 12 and also to over etch into thesilicon wafer 10 by a distance of approximately 500 Å.

[0039] The mask 14 shown in FIGS. 2A and 2B is then removed and anotherphotoresist layer 16 is applied which is patterned as shown in FIGS. 3Aand 3B by providing two openings therein 16-1 and 16-2. Opening 16-1basically follows the outer perimeter of the ‘E’ shape of the underlyingthin layer of p-type silicon 12 while opening 16-2 is disposed at oradjacent a tip of the interior leg of the ‘E’-shaped p-type siliconlayer 12.

[0040] Layers of Ti/Pt/Au are next deposited over mask 16 and throughopenings 16-1 and 16-2 to form a post contact 18-1 and a tunnelling tipcontact 18-2. The Ti/Pt/Au layers preferably have a total thickness ofabout 2000 Å. The individual layers of Ti and Pt may have thicknesses inthe ranges of 100-200 Å and 1000-2000 Å, respectively. After removal ofthe photoresist 16, the wafer is subjected to a sintering step atapproximately 520° C. to form an ohmic Ti—Si juncture between contacts18-1 and 18-2 and the underlying layer 12.

[0041] The structures shown in FIGS. 4A and 4B are then covered with alayer of photoresist 20 which, as shown in FIG. 5A, is patterned so thatis assumes the same shape as did photoresist layer 16 previouslydiscussed with reference to FIGS. 3A and 3B. Thus, photoresist layer 20has an opening 20-1 and another opening 20-2 therein. Those skilled inthe art will appreciate that the size of the openings 16-1, 16-2, 20-1and 20-2 are not drawn to scale on the figures and that openings 16-2and 20-2 would tend to be significantly smaller than would be openings16-1 and 20-1. As such, when a rather thick layer of Ti/Pt/Au isdeposited on the wafer, it basically fills opening 20-1 (see FIG. 5A);however, those skilled in the art will appreciate that there is somefill-in at the sides of a mask when a layer such as layer 22 isdeposited because of an increasing overhang which occurs at the edges ofopenings 20-1 and 20-2 as the deposition process proceeds. Since thewidth of the opening 20-1 is quite wide, the effect of the fill-in isnot particularly important. However, since opening 20-2 is rather narrowto begin with, the deposited Ti/Pt/Au 22, as shown at numeral 20-2,assumes a pyramidal-like or conical-like shape. The thickness of thedeposition of Ti/Pt/Au layer 22 is sufficiently thick to assure thatlayer 22 will close across the top of opening 20-2 during the depositionprocess. Finally, a relatively thin layer, preferably about 100Å thick,of Au/Si 24 is deposited on the structure and through opening 20-1 asdepicted by numeral 24-1.

[0042] The photoresist 20 is then dissolved lifting of the layers 22 and24 formed thereon and leaving the structure depicted by FIGS. 6A and 6B.The height of layers 22-1 and 24-1 above layer 12 is preferably on theorder of 11,500 Å while the height of the pyramidal-like or conicalstructure 22-2 is preferable on the order of 8,500 Å. The cantileveredbeam portion of the MEMS sensor of this first embodiment has now beenformed, and thus we will now move onto the formation of the basestructure for this first embodiment of the MEM sensor. As will be seen,layers 22-1 and 24-1 form a mating structure for mating the cantileveredbeam portion with its base portion.

[0043] The fabrication of the base portion will now be described.Turning first to FIGS. 7A and 7B, there is shown a wafer 30 with layersof silicon dioxide 32 and 34 formed on its major surfaces. The thicknessof each layer 32 and 34 is preferably on the order of 1.0 micrometers.Next, a mask is formed by a layer of photoresist 36 which is applied andpatterned as shown in FIGS. 8A and 8B to form openings 36-1, 36-2, 36-3and 36-4 therein. Opening 36-1 basically corresponds in shape andconfiguration to opening 16-1 discussed with reference to FIGS. 3A and3B. Similarly, opening 36-2 basically corresponds to opening 16-2discussed with reference to FIGS. 3A and 3B. Openings 36-3 and 36-4allow for the deposition of control and self test electrodes 38-3 and38-4. A layer of Ti/Pt/Au 38 is deposited on mask 36 and through theopenings therein in order to form contact electrodes 38-1, 38-2, 38-3and 38-4 on layer 34. Photoresist layer 36 also has openings in it sothat when layer 38 is deposited, connection pads 40 are also formed foreach one of the electrodes as well as interconnecting ribbon conductors42. Preferably, a guard ring 44 is placed around tip electrode 36-2 andits associated ribbon conductor 42-2 and connection pad 40-2. The guardring is not shown in the elevation views for ease of illustration.

[0044] The photoresist layer 36 is then removed lifting off the layer 38deposited thereon and leaving the structure shown in FIGS. 9A and 9B.Contact 38-1 assumes the shape of the outer periphery of a letter E andprovides a mating structure for joining with the similar-shaped matingstructure 22-1, 24-1 of the cantilevered beam portion 2.

[0045] Turning to FIG. 10, the cantilevered beam portion 2, preferablyfabricated as described with reference to FIGS. 1A-6B, is mechanicallyaligned relative to the base portion 4 fabricated as described withreference to FIGS. 7A-9B. Of course, those skilled in the art willappreciate that the patterns shown on the surfaces of wafers 10 and 30repeat many times over the surface of a wafer so that there are manycantilevered beam forming structures 2 comprising elements 221, 24-1, 12and 22-2 and many corresponding base structures 4 comprising elements38-1 through 38-4 which are manufactured for mating on the siliconwafers 10 and 30. The two wafers are brought into alignment (See FIG.11) and subjected to pressure and heating so as to cause a eutectic bondto occur between layer 24-1 and layer 38-1. The pressure is developedpreferably by applying a force of about 5,000 N at about 400° C. betweenthree inch (7.5 cm) wafers 2,4 containing 1000 devices. Of course, theforce needs to be adjusted depending n the size of the wafer and thetotal surface area to be bonded. If the bonding is donenon-eutectically, the temperature used will be higher.

[0046] Layers 24-1 and 38-1 have preferably assumed the shape of theouterperpherial edge of a capital letter ‘E’ and therefore the moveablecontact 22-2 of the MEM sensor is well protected by this physical shape.After performing the bonding, silicon layer 10 is dissolved away toarrive at the resulting MEM sensor shown in FIGS. 12A and 12B. Thesilicon can be dissolved with ethylenediamine pyrocatechol (EDP). Thisleaves only the Boron doped silicon cantilevered beam 12 with itscontact 22-2 and its supporting or mating structure 22-1 and 24-1 bondedto the base structure 4. Preferable dimensions for the MEM sensor aregiven on FIG. 12A. The beam preferably has a length of 200 to 300 μm(0.2 to 0.3 mm).

[0047] Instead of using EDP as the etchant, plasma etching can be usedif a thin layer of SiO₂ is used, for example, as an etch stop betweenlayer 12 and substrate 10.

[0048] A second embodiment of a MEM sensor will now be described. As inthe case of the first embodiment, this discussion will begin with thefabrication of the cantilever beam portion 2, then go onto a discussionof the base portion 4 and the preferable eutectic bonding and thecompletion of the MEM sensor. As will be seen, this second embodimentdiffers from the first embodiment by the manner in which thecantilevered beam is supported above base portion 4.

[0049] According to the second embodiment, the fabrication of thecantilever beam forming structure 2 starts as has been described withreference to FIGS. 1A through 4B of the first embodiment. Assuming thatthe fabrication steps discussed with reference to FIGS. 1A through 4Bhave been carried out, the structure depicted in FIGS. 4A and 4B willbeen obtained.

[0050] Alternatively, post contact 18-1 may be formed by layers of Tiand Au (i.e without Pt), which would involve an additional masking stepto eliminate the Pt layer from post contact 18-1. However, in thisalternative, the sintering would cause Si to migrate into the Au to forman Au/Si eutectic at the exposed portion of post contact 18-1 shown inFIGS. 4A and 4B. As a further alternative, the exposed portion of thepost contact 18-1 shown in FIGS. 4A and 4B could simply be deposited asAu/Si eutectic, in which case the Pt layer in the post contact 18-1could be optionally included. Post contact 18-1 may be eliminated if thesubsequently described bonding between the cantilevered beam portion 2and the base portion 4 occurs non-eutectically.

[0051] As a result, the exposed portion of the post contact 18-1 shownin FIGS. 4A and 4B is formed, according to this embodiment preferablyeither by Au or by Au/Si.

[0052] From that point, a layer of photoresist 20′ is put down andpatterned to have a single opening 20-2 therein as shown in FIGS. 13Aand 13B. A layer of gold 26, having a thickness of 15,000 Å, is appliedover the photoresist 20′ and the gold, as it deposits upon contact 18-2through opening 20-2, will assume a pyramidal-like or conical-like shapeso as to form a pointed contact 26-2 due to the formation of an overhangat the opening 20-2 during the deposition of the gold layer 26. Aftercontact 26-2 is formed, the remaining photoresist 20′ is dissolved sothat the cantilever beam structure then appears as shown in FIGS. 14Aand 14B. Comparing FIGS. 14A and 14B of the second embodiment with FIGS.6A and 6B of the first embodiment, the primary difference between thetwo embodiments is the absence of layers 22-1 and 24-1 in the secondembodiment, so that the mating structure is provided by layer 18-1 inthis embodiment.

[0053] The fabrication of the base portion 4 of the second embodiment ofthe MEM sensor will now be described with reference to FIGS. 15A through19B. Turning to FIGS. 15A and 15B, a wafer 30′ of silicon is shown uponwhich a layer of photoresist S0 has been deposited and patterned toassume preferably the outerperipheral shape of a capital letter ‘E’. Theexposed silicon is then subjected to an etch, etching it backapproximately 20,000 Å, to define a protruding portion 30-1 of wafer 30′under the patterned mask 50 of the photoresist. The photoresist mask 50is then removed and wafer 30 is oxidized to form layers of oxide 52, 54on its exposed surfaces. The oxide layers are each preferably about 1 μmthick. Of course, the end surfaces shown in FIG. 16A are not shown asbeing oxidized because it is assumed that the pattern shown in FIG. 16Ais only one of a number of repeating patterns occurring across an entirewafer 30′.

[0054] Turning to FIGS. 17A and 17B, a layer of photoresist 56 isapplied having an opening therein 56-1 which again assumes theouterperipheral shape of a capital letter ‘E’, as previously described.Then, a layer of Ti/Pt/Au 58, preferably having a thickness of 2,000 Å,is deposited through opening 56-1 followed by the deposition of a layer60 of an Au/Si eutectic preferably with a 1,000 Å thickness. Layers 58-1of Ti/Pt/Au and 60-1 of the Au/Si eutectic are thus formed, which layerspreferably follow the outerperipheral shape of a capital letter ‘E’, aspreviously described. Of course, if the post contact 18-1 is formed ofan Au/Si eutectic, then layer 60 may be formed of simply Au or simplyomitted due to the presence of Au at the exposed layer 58-1.

[0055] Photoresist layer 56 is then removed and a layer 62 ofphotoresist is applied and patterned to have (i) openings 62-2, 62-3 and62-4, as shown in FIG. 18A, (ii) openings for pads 40-1 through 40-4 andtheir associated ribbon conductors 42 and (iii) an opening for guardring 44 and its pad, as depicted in FIG. 18B. For the ease ofillustration, the opening for guard ring 44 is not shown in FIG. 18A. Asis shown by FIGS. 19A and 19B, a layer 38 of Ti/Pt/Au is deposited overthe photoresist and through openings 62-2 through 62-4 forming contacts38-3, 38-4 and 38-2 as shown in FIGS. 19A and 19B. Those contacts areinterconnected with their associated pads 40-2 through 44-4 by theaforementioned ribbon conductors 42, which contacts 40 and ribbonconductors 42 are preferably formed at the same time as contacts 38-3,38-4 and 38-2 are formed. The outerperipheral layers 58-1 and 60-1 arealso connected with pad 40-1 by an associated ribbon conductor 42. Thelayer 62 of photoresist is removed so that the base portion appears asshown in FIGS. 19A and 19B. The protrusion 30-1, which preferablyextends approximately 20,000 Å high above the adjacent portions of wafer30′, and the relatively thin layers 58-1 and 60-1 form the matingstructure for the base portion 4.

[0056] Turning to FIG. 20, the cantilever beam forming portion 2according to the second embodiment is now bonded to base portion 4. Asis shown in FIG. 20, the two wafers 10 and 30′ are brought into aconfronting relationship so that their mating structure 18-1, 30-1, 58-1and 60-1 are in alignment so that layers 18-1 and 60-1 properly matewith each other. Pressure and heat (preferably by applying a force of5,000 N at 400° C. between three inch wafers 2, 4 having 1000 sensorsdisposed thereon) are applied so that eutectic bonding occurs betweenlayers 18-1 and 60-1 as shown in FIG. 21. Thereafter, silicon wafer 10is dissolved so that the MEM sensor structure shown in FIG. 22 isobtained. The p-type silicon layer 12 includes a portion 12-2 whichserves as the cantilevered beam and another portion which is attached tothe base portion 4 through the underlying layers. The gold contact 26-2is coupled to pad 40-1 by elements 18-2, 12-2, 12-1, 18-1, 60-1, 58-1and its associated ribbon conductor 42. If the bonding is donenon-eutectically, then higher temperatures will be required.

[0057]FIG. 23 is basically identical to FIG. 22, but shows the MEMsensor in somewhat more detail and the preferred dimensions of the MEMsensor are also shown on this figure.

[0058] It will be recalled that in both the first and second embodimentdiscussed above with respect to FIG. 4B, a layer of Ti/Pt/Au 18 wasapplied forming contacts 18-1 and 18-2 which were sintered in order toform an ohmic bond with Boron-doped cantilever 12. It was noted thatsintering could be avoided by providing a ribbon conductor betweencontacts 18-1 and 18-2. Such a modification is now described in greaterdetail and is depicted starting with FIGS. 24A and 24B.

[0059] According to this modification, the Si epitaxial layer 12 formedon silicon wafer 10 may be (i) doped with Boron or (ii) may be eitherundoped or doped with other impurities. If undoped (or doped with otherimpurities), then a thin etch stop layer 11 is used between the Sidevice layer 12 and the silicon wafer 10. This configuration is calledSilicon On Insulator (SOI). The etch stop layer 11, if used, ispreferably a layer of SiO₂ having a thickness of about 1-2 μm. This etchstop layer 11 will be used to release the cantilevered beam from wafer10. If layer 12 is doped with Boron, it is doped to reduce theresistivity of the epitaxial layer 12 to less than 0.05 Ω-cm. At thatlevel of doping the epitaxial layer 12 can resist a subsequent EDP etchused to release the cantilevered beam from wafer 10.

[0060] Optionally, the silicon wafer 10 with the doped or undoped Siepitaxial layer 12 formed thereon (as shown in FIGS. 24A and 24B) may besubjected to thermal oxidation to form a relatively thin layer of SiO₂on the exposed surface of layer 12. Layer 12 is preferably about 1.2 μmthick (but it can be thinner or thicker depending upon the application).The thickness of the optional SiO₂ layer is preferably on the order of0.2 μm. To arrive at this point, both major surfaces may be oxidized andthe oxide stripped from the bottom layer, if desired. The optional oxidelayer may be used to provide an even better barrier against thediffusion of Si from the beam into the Au of the tunneling tip formed atone end of the beam. This optional oxide layer may be used with anyembodiment of the cantilevered beam, but is omitted from most of thefigures for case of illustration. It does appear, however, in FIGS. 37and 41 and is identified there by element number 70.

[0061] Turning now to FIGS. 25A and 25B, a layer of photoresist 14 isthen applied on layer 12 (or on the optional oxide layer, if present)and patterned preferably to assume the same “E” letter shape as thelayer photoresist 14 discussed with reference to FIGS. 2A and 2B. Thestructure shown in FIGS. 25A and 25B is then subjected to a plasma etchwhich etches through layers 11 and 12 into the silicon substrate 10 byapproximately 500 Å. Then a layer of photoresist 16 is applied andpatterned as shown by FIGS. 26A and 26B. The layer 16 of photoresist ispatterned to assume basically the same arrangement and configuration aslayer 16 discussed with respect to FIGS. 3A and 3B except that anadditional opening 16-5 is included communicating between openings 61-1and 16-2 to provide for the formation of a ribbon conductor 18-5 when alayer 18 of metals, preferably Ti/Pt/Au, is subsequently deposited onphotoresist 16. After depositing the layer 18, the photoresist 16 isremoved lifting off the portions of the layer 18 formed thereon, leavingportions 18-1, 18-2 and 18-5 of layer 18 on the underlying layer 12, asshown in FIGS. 27A and 27B, or on the optional oxide layer, if present.

[0062] After arriving at the structure shown in FIGS. 27A and 27B, atunneling tip 22 is added by appropriate masking and deposition of Au ora layer of Ti/Pt/Au, for example, thereby arriving at the structureshown by FIGS. 28A and 28B. Depending on the configuration utilized, amember 22-1 (see FIG. 6A) could be deposited at the same time so thatthe MEM sensor would be completed as previously described with referenceto FIGS. 10 and 11. If instead the silicon base 30 is formed with aprotrusion 30-1 (see FIG. 16A, for example), then the deposition ofmember 22-1 can be omitted and the MEM sensor can be completed aspreviously described with reference to FIGS. 20 and 21. After bondingthe structure depicted by FIGS. 28A and 28B to the base structure 4 ofFIGS. 19A and 19B and releasing the silicon wafer 10 from thecantilevered beam, the structure shown by FIGS. 29A and 29B is arrivedat. The cantilevered beam 12 is preferably released by performing twoplasma etches. The first etch dissolves wafer 10 and the second etchremoves the etch stop layer 11.

[0063]FIG. 30 shows yet another embodiment of a MEM sensor. In this casethe MEM sensor is shown in its completed form. With the informationalready presented herein, those skilled in the art will not find itdifficult to modify the detailed description already given to producethis embodiment and still further embodiments, all of which will now bediscussed. In the embodiment of FIGS. 1A-12B the preferable eutecticbond occurs close to the silicon substrate 10 between layers 24-1 and38-1. In the embodiment of FIG. 30, the preferable eutectic bond occurscloser to a center point in the supporting arm 80 between the Au andAu/Si layers. Otherwise this embodiment is similar to the firstembodiment described with reference to FIGS. 1A-12B. In the embodimentof FIG. 31, the preferable eutectic bond occurs between the Au and Au/Silayers which are arranged close to the cantilevered beam 12 as opposedto close to base 4 as in the case of the first embodiment described withreference to FIGS. 1A-12B. In the case of the embodiments of FIGS. 30and 31, the cantilevered beam 12 should have good conductivity so thatit acts as a conduction path between contact 22-2 at the end of the beam12 and contact 40-1 on the base 4 (See FIG. 12B). Preferably theresistivity of the boron doped silicon cantilevered beam 12 is less than0.05 Ω-cm. Due to the low resistivity of the beam 12, EDP may be used toetch away substrate 10 (see FIGS. 10 and 11 and the related description)Alternatively, an SOI wafer could be used and the SiO₂ layer 11 (FIGS.24A-28B) would be used as an etch stop layer to protect the beam 12 whenetching away substrate 10.

[0064] Comparing the embodiments of FIGS. 10, 23, 29 and 30, theembodiments of FIGS. 23 and 29 are preferred since they only need arelatively thin metal mating layer and provide a more rigid Si post orprotrusion 30-1 for better stability.

[0065] The embodiments of FIGS. 32 and 33 are similar to the embodimentsof FIGS. 29 and 30, but these two embodiments make use of the ribbonconductor 18-5 described with reference to FIGS. 24A through 29B. Forthese embodiments, if layer is doped with Boron, the resistivity of thecantilevered beam 12 is preferably less than 0.05 Ω-cm. The ribbonconductor allows the use of higher resistivity silicon for thecantilevered beam 12. If layer 12 is doped with Boron, then thecantilevered beam can be released from wafer 10 using EDP as theetchant. Alternatively, an SIO construction can be utilized with a SiO₂stop layer 11 (See FIGS. 24A-28B) utilized to protect the beam 12 whilethe substrate 10 is etched away.

[0066] The embodiments of FIGS. 34-37 are similar to the embodiment ofFIGS. 29, 31, 30 and 32, respectively, except instead of using a planarsubstrate, a substrate with a silicon protrusion 30-1 is utilized asdescribed with reference to the second embodiment (see FIGS. 13A-23 andthe related description).

[0067] Generally speaking, the embodiments of FIGS. 13A-23 and 34-37 arepreferred for a MEM sensor since these embodiments, which all utilizethe a base substrate 30′ with a silicon post or protrusion 30-1, arebelieved to give the resulting sensors and switches better mechanicalstability.

[0068] The structure which has been described so far has been set up asa sensor. Those skilled in the art know not only how to utilize thesestructures as a sensor but also know how to modify these structures,when needed, to make them function as a switch. The sensor devices shownin the preceding figures are preferably used as accelerometers, althoughthey can be used for other types of sensors (such as gyroscopes,magnetometers, etc.) or as switches, as a matter of design choice, andwith appropriate modification when needed or desired.

[0069] Four embodiments of a switch version of a MEM device inaccordance with the present invention will now be described withreference to FIGS. 38-41. In order to function as a switch, two metalpads 26-3 and 26-4 are deposited on the cantilevered beam structure 12instead of a pointed contact 26-2. In these embodiments the cantileveredbeam is preferably formed of undoped silicon. When the switch closes,the metal pad 26-4 bridges two contacts 38-5 and 38-6, which aredeposited at the same time that layer 38 is deposited on the basestructure 4. The ribbon conductor 18-5 described with reference to FIGS.24A through 29B is utilized, due to the relatively high resistivity ofundoped Si, to bring an electrical connection with metal pad 26-3 downto the base substrate 4. The switch is closed by imparting anelectrostatic force on the cantilevered beam 12 by applying a voltagebetween metal pads 38-3 and 26-3. That voltage causes the metal pad 26-4to make a circuit connecting contacts 38-5 and 38-6 when the metal pad26-4 makes physical contact with those two contacts when the switchcloses. Otherwise these embodiments are similar to the previouslydiscussed embodiments. It should be noted, however, that since thecantilevered beam 12 is preferably formed of undoped silicon, the EDPetchant will not prove satisfactory. Instead the SiO₂ etch stop layer 11described with reference to FIGS. 24A-29B is preferably used to protectthe beam 12 when etching away substrate 10.

[0070] In FIG. 38 the switch is formed on a generally planar base 4 andthe cantilevered beam is supported by a column 80 formed of depositedmetals and the Au/Si eutectic. The Au/Si eutectic is arranged towardsthe middle of the column in this embodiment. In the embodiment of FIG.39 the Au/Si eutectic is arranged closer to the beam 12. In theembodiment of FIG. 40 the Au/Si eutectic layer is disposed next to thebeam and in this embodiment the base structure 4 has a protrusion 30-1which acts as a portion of the column 80 which supports the beam 12. Ofthe switch embodiments, the embodiment of FIG. 40 is preferred for thesame reason that sensors with a protrusion 30-1 in their base structures4 are also preferred, namely, it is believed to give the resultingsensors and switches better mechanical stability.

[0071] In FIG. 41 an SiO₂ layer 70 is shown disposed between beam 12 andlayer 18. Layer 18 preferably is formed of layers of Ti, Pt and Au. ThePt acts as a diffusion barrier to the Si to keep it from migrating intothe Au contacts. If layer 18 does not provide adequate protection forwhatever metal is used in making contacts, then the use of a diffusionbarrier such a SiO₂ layer 70 would be appropriate.

[0072] The structures shown in the drawings has been described in manyinstances with reference to a capital letter ‘E’. However, this shape isnot particularly critical, but it is preferred since it provides goodmechanical support for the cantilevered structure formed primarily bybeam portion of layer 12. Of course, the shape of the supportingstructure or mating structure around cantilever beam 12 can be changedas a matter of design choice and it need not form the perimeter of thecapital letter ‘E’, but can form any convenient shape, includingcircular, triangular or other shapes as desired.

[0073] In the embodiment utilizing a ribbon conductor on thecantilevered beam 12, the pads and contacts (e.g. 26-2 and 26-3) formedon the beam 12 are generally shown as being formed over the ribbonconductor 18-1, 18-2, 18-5. The ribbon conductor on the beam can berouted in any convenient fashion and could butt against or otherwisemake contact with the other metal elements formed on the cantileveredbeam 12 in which case elements such as 26-2 and 26-3 could be formeddirectly on the beam 12 itself.

[0074] The contacts at the distal ends of the cantilevered beams aredepicted and described as being conical or triangular. Those skilled inthe art will appreciate that those contacts may have otherconfigurations and may be flat in some embodiments.

[0075] Throughout this description are references to Ti/Pt/Au layers.Those skilled in the art will appreciate that this nomenclature refersto a situation where the Ti/Pt/Au layer comprises individual layers ofTi, Pt and Au. The Ti layer promotes adhesion, while the Pt layer actsas a barrier to the diffusion of Si from adjacent layers into the Au.Other adhesion layers such as Cr and/or other diffusion barrier layerssuch as a Pd could also be used or could alternatively be used. It isdesirable to keep Si from migrating into the Au, if the Au forms acontact, since if Si diffuses into an Au contact it will tend to formSiO₂ on the exposed surface and, since SiO₂ is a dielectric, it hasdeleterious effects on the ability of the Au contact to perform itsintended function. As such, a diffusion barrier layer such as Pt and/orPd is preferably employed between an Au contact and adjacent Simaterial.

[0076] The nomenclature Au/Si or Au—Si refers a mixture of Au and Si.The Au and Si can be deposited as separate layers with the understandingthat the Si will tend to migrate at elevated temperature into the Au toform an eutectic. However, for ease of manufacturing, the Au/Si eutecticis preferably deposited as a mixture except in those embodiments wherethe migration of Si into Au is specifically relied upon to form Au/Si.

[0077] Many different embodiments of a MEM device have been described.Most are sensors and some are switches. Many more embodiments cancertainly be envisioned by those skilled in the art based the technologydisclosed herein. But in all cases the base structure 4 is united withthe cantilevered beam forming structure 2 by applying pressure andpreferably also heat, preferably to cause an eutectic bond to occurbetween the then exposed layers of the two structures 2 and 4. Thebonding may instead be done non-eutectically, but then highertemperatures must be used. Since it is usually desirable to reduceand/or eliminate high temperature fabrication processes, the bondingbetween the two structures 2 and 4 is preferably done eutectically andthe eutectic bond preferably occurs between confronting layers of Si andAu/Si.

[0078] Having described the invention with respect to certain preferredembodiments thereof, modification will now suggest itself to thoseskilled in the art. The invention is not to be limited to the foregoingdescription, except as required by the appended claims.

What is claimed is:
 1. A method of making a MEM tunneling sensorcomprising the steps of: (a) defining a cantilevered beam structure anda mating structure on a first substrate or wafer; (b) forming at leastone contact structure and a mating structure on a second substrate orwafer, the mating structure on the second substrate or wafer being of acomplementary shape to the mating structure on the first substrate orwafer; (c) positioning the mating structure of the first substrate orwafer into a confronting relationship with the mating structure of thesecond substrate or wafer; (d) bonding a layer associated with saidmating structure on the first substrate or wafer with a layer associatedwith the mating structure on the second substrate or wafer; and (e)removing at least a portion of the first substrate or wafer to releasethe cantilevered beam structure.
 2. A method of making a MEM tunnelingsensor as claimed in claim 1 wherein the second substrate or wafer isformed of silicon.
 3. A method of making a MEM tunneling sensor asclaimed in claim 2 wherein the silicon forming the second substrate orwafer is of a single crystalline structure.
 4. A method of making a MEMtunneling sensor as claimed in claim 3 wherein the crystalline structureof the silicon is <100>.
 5. A method of making a MEM tunneling sensor asclaimed in claim 4 wherein the silicon is n-type.
 6. A method of makinga MEM tunneling sensor as claimed in claim 1 wherein the first substrateor wafer is formed of silicon.
 7. A method of making a MEM tunnelingsensor as claimed in claim 6 wherein the silicon forming the firstsubstrate or wafer is of a single crystalline structure.
 8. A method ofmaking a MEM tunneling sensor as claimed in claim 7 wherein thecrystalline structure of the silicon in the first substrate or wafer is<100>.
 9. A method of making a MEM tunneling sensor as claimed in claim8 wherein the silicon of the first substrate or wafer is n-type.
 10. Amethod of making a MEM tunneling sensor as claimed in claim 1 whereinheat is applied together with pressure between the two substrates so asto cause an eutectic bond to occur between the two mating structures.11. A method of making a MEM tunneling sensor as claimed in claim 1wherein the cantilevered beam structure is formed by: (a) forming anepitaxial layer of silicon on said first substrate or wafer, saidepitaxial layer being doped; (b) masking and etching the epitaxial layerof silicon to define a beam structure disposed on said first substrateor wafer; and (c) removing the first substrate or wafer by etching. 12.A method of making a MEM tunneling sensor as claimed in claim 11 whereina contact is formed on an end of said beam structure by depositing ametal through a small opening in a temporary mask layer, the smallopening being sufficiently small that the metal being deposited tends tooverhang the small opening increasingly as the deposition of the metalproceeds whereby the contact being deposited through the small openingassumes an elongate shape of decreasing cross section as the depositionproceeds.
 13. A method of making a MEM tunneling sensor as claimed inclaim 11 wherein etching accomplished by ethylenediamine pyrocatechol asan etchant.
 14. A method of making a MEM tunneling sensor as claimed inclaim 11 wherein the epitaxial layer is doped with boron at a sufficientconcentration to reduce the resistivity of the epitaxial layer to lessthan 0.05 Ω-cm.
 15. A method of making a MEM tunneling sensor as claimedin claim 14 wherein a layer of metal is selectively deposited on saidepitaxial layer and sintered at an elevated temperature to form firstand second ohmic contacts on said epitaxial layer, said second ohmiccontact being disposed near a distal end of the beam structure and thefirst ohmic contact forming the mating structure on the first substrateor wafer.
 16. A method of making a MEM tunneling sensor as claimed inclaim 15 wherein the layer of metal is formed of individual layers ofTi, Pt and Au.
 17. A method of making a MEM tunneling sensor as claimedin claim 16 wherein a relatively thick layer of Ti/Pt/Au is depositedand then sintered on a relatively thin metal layer of Ti/Pt/Au, a firstportion of the relatively thick layer of Ti/Pt/Au forming the matingstructure on the first substrate or wafer and overlying said first ohmiccontact and a second portion of the relatively thick layer of metalforming a pointed contact at said second ohmic contact.
 18. A method ofmaking a MEM tunneling sensor as claimed in claim 17 further includingforming Ti/Pt/Au contacts on said second substrate or wafer, at leastone of said contacts on the second substrate or wafer defining themating structure on the second substrate or wafer.
 19. A method ofmaking a MEM tunneling sensor as claimed in claim 18 wherein the bondingoccurs eutectically and the layer for producing an eutectic bond isprovided by a layer of Au—Si eutectic deposited on the Ti/Pt/Au contacton said second substrate or wafer and/or by a layer of Au—Si eutecticdeposited on first portion of the relatively thick layer of Ti/Pt/Au onthe first substrate or wafer.
 20. A MEM tunneling sensor assembly formaking a MEM tunneling sensor therefrom, the assembly comprising: (a) abeam structure and a mating structure defined on a first substrate orwafer; (b) at least one contact structure and a mating structure definedon a second substrate or wafer, the mating structure on the secondsubstrate or wafer being of a complementary shape to the matingstructure on the first substrate or wafer; and (c) a pressure/heatsensitive bonding layer disposed on at least one of said matingstructures for bonding the mating structure defined on the firstsubstrate or wafer to mating structure on the second substrate or waferin response to the application of pressure/heat therebetween.
 21. A MEMtunneling sensor assembly as claimed in claim 20 wherein the secondsubstrate or wafer is formed of silicon.
 22. A MEM tunneling sensorassembly as claimed in claim 21 wherein the silicon forming the secondsubstrate or wafer is of a single crystalline structure.
 23. A MEMtunneling sensor assembly as claimed in claim 22 wherein the crystallinestructure of the silicon is <100>.
 24. A MEM tunneling sensor assemblyas claimed in claim 23 wherein the silicon is n-type.
 25. A MEMtunneling sensor assembly as claimed in claim 20 wherein the firstsubstrate or wafer is formed of silicon.
 26. A MEM tunneling sensorassembly as claimed in claim 25 wherein the silicon forming the firstsubstrate or wafer is of a single crystalline structure.
 27. A MEMtunneling sensor assembly as claimed in claim 26 wherein the crystallinestructure of the silicon in the first substrate or wafer is <100>.
 28. AMEM tunneling sensor assembly as claimed in claim 27 wherein the siliconof the first substrate or wafer is n-type silicon.
 29. A MEM tunnelingsensor assembly as claimed in claim 20 wherein a pointed contact isdisposed on an end of said beam structure.
 30. A MEM tunneling sensorassembly as claimed in claim 29 wherein the epitaxial layer is dopedwith Boron at a sufficient concentration to reduce the resistivity ofthe epitaxial layer to less than 0.05 Ω-cm.
 31. A MEM tunneling sensorassembly as claimed in claim 30 further including first and second ohmiccontacts on said epitaxial layer, said second ohmic contact beingdisposed near a distal end of the beam structure and said first ohmiccontact forming the mating structure on the first substrate or wafer.32. A MEM tunneling sensor assembly as claimed in claim 31 wherein thefirst and second ohmic contacts are formed of layers of Ti, Pt and Au.33. A MEM tunneling sensor assembly as claimed in claim 32 wherein arelatively thick layer of Ti/Pt/Au is disposed on the first and secondohmic Ti/Pt/Au contacts , a first portion of the relatively thick layerof Ti/Pt/Au being disposed on said first ohmic Ti/Pt/Au contact andproviding the mating structure on the first substrate and a secondportion of the relatively thick layer of Ti/Pt/Au forming a pointedcontact on said second ohmic Ti/Pt/Au contact.
 34. A MEM tunnelingsensor assembly as claimed in claim 32 further including Ti/Pt/Aucontacts disposed on said second substrate or wafer, at least one ofsaid contacts on the second substrate or wafer defining the matingstructure on the second substrate or wafer.
 35. A MEM tunneling sensorassembly as claimed in claim 34 wherein the bonding layer is provided bya layer of Au—Si eutectic disposed on the Ti/Pt/Au contact on saidsecond substrate and/or by a layer of Au—Si eutectic disposed on thefirst portion of the relatively thick layer of Ti/Pt/Au on the firstsubstrate or wafer.
 36. A MEM tunneling sensor assembly as claimed inclaim 31 further including first and second ohmic contacts on saidepitaxial layer, said second ohmic contact being disposed near a distalend of the beam structure and said first ohmic contact forming themating structure on the first substrate.
 37. A MEM tunneling sensorassembly comprising: (a) a beam structure and a mating structure definedon a first substrate or wafer; (b) at least one contact structure and amating structure defined on a second substrate or wafer, the matingstructure on the second substrate or wafer being of a complementaryshape to the mating structure on the first substrate or wafer; and (c) abonding layer is disposed on at least one of said mating structures forbonding the mating structure, defined on the first substrate or wafer tothe mating structure on the second substrate or wafer, the matingstructures being joined one to another at said bonding layer.
 38. A MEMtunneling sensor assembly as claimed in claim 37 wherein the first andsecond substrates or wafers are each formed of single crystal silicon.39. A MEM tunneling sensor assembly as claimed in claim 38 wherein thecrystalline structure of the silicon is <100>.
 40. A MEM tunnelingsensor assembly as claimed in claim 37 wherein the cantilevered beamstructure is formed from an epitaxial layer of silicon on said firstsubstrate or wafer, said epitaxial layer being doped with a dopant. 41.A MEM tunneling sensor assembly as claimed in claim 40 wherein theepitaxial layer is doped with Boron at a sufficient concentration toreduce the resistivity of the epitaxial layer to less than less than0.05 Ω-cm.
 42. A MEM tunneling sensor assembly as claimed in claim 40further including first and second ohmic contacts on said epitaxiallayer, said second ohmic contact being disposed near a distal end of thebeam structure and said first ohmic contact forming the mating structureon the first substrate or wafer.
 43. A MEM tunneling sensor assembly asclaimed in claim 40 wherein the first and second ohmic contacts areformed of layers of Ti, Pt and Au.
 44. A MEM tunneling sensor assemblyas claimed in claim 42 wherein a relatively thick layer of metal isdisposed on the first and second ohmic contacts, a first portion of therelatively thick layer of metal being disposed on said first ohmiccontact and providing the mating structure on the first substrate orwafer and a second portion of the relatively thick layer of metalforming a pointed contact on said second ohmic contact.
 45. A MEMtunneling sensor assembly as claimed in claim 44 further including metalcontacts disposed on said second substrate or wafer, at least one ofsaid contacts on the second substrate or wafer defining the matingstructure on the second substrate or wafer.
 46. A MEM tunneling sensorassembly as claimed in claim 45 wherein the bonding layer is provided bya layer of Au—Si eutectic disposed on the metal contact on said secondsubstrate or wafer and/or by a layer of Au—Si eutectic disposed on thefirst portion of the relatively thick layer of metal on the firstsubstrate or wafer.
 47. A MEM tunneling sensor assembly as claimed inclaim 42 further including first and second ohmic contacts on saidepitaxial layer, said second ohmic contact being disposed near a distalend of the beam structure and said first ohmic contact forming themating structure on the first substrate or wafer.